The invention relates to a two-stage process for reactivating thermally aged nitrogen oxide storage catalysts which comprise nitrogen oxide-storing compounds on a support material comprising cerium oxide and platinum as a catalytically active noble metal.
Nitrogen oxide storage catalysts are used to remove the nitrogen oxides present in the lean offgas of lean-burn engines. The cleaning effect is based on, in a lean operating phase of the engine, storage of the nitrogen oxides by the storage material of the storage catalyst, predominantly in the form of nitrates, and, in a subsequent rich operating phase of the engine, decomposition of the nitrates formed beforehand and reaction of the nitrogen oxides released again with the reducing exhaust gas constituents over the storage catalyst to give nitrogen, carbon dioxide and water. The lean-burn engines include gasoline and diesel engines which can be operated with a lean air/fuel mixture. The nitrogen oxides present in the exhaust gas of these engines during the lean phases consist mainly of nitrogen monoxide.
The way in which nitrogen oxide storage catalysts work is described in detail in the SAE document SAE 950809. According to this, nitrogen oxide storage catalysts consist of a catalyst material which is usually applied in the form of a coating to a support body. The catalyst material comprises a nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material in turn consists of the actual nitrogen oxide storage component, which is deposited on a support material in highly disperse form.
The storage components used are principally the basic oxides of the alkali metals, of the alkaline earth metals and of the rare earth metals, but especially strontium oxide and barium oxide, which react with nitrogen dioxide to give the corresponding nitrates. It is known that these materials are present under air predominantly in the form of carbonates and hydroxides. These compounds are likewise suitable for storing the nitrogen oxides. When reference is therefore made in the context of the invention to the basic stored oxides, this also includes the corresponding carbonates and hydroxides.
Suitable support materials for the storage components are thermally stable metal oxides with a high surface area of more than 10 m2/g, which enable highly disperse deposition of the storage components. The present invention is concerned especially with storage materials which have cerium oxide-containing support materials. These include doped cerium oxides and particularly cerium-zirconium mixed oxides, which may likewise be doped.
The catalytically active components used are the noble metals of the platinum group, which may be present separately from the storage components on a separate support material. The support material used for the platinum group metals is usually active, high-surface area aluminum oxide, which may likewise comprise doping components.
The task of the catalytically active components is to convert carbon monoxide and hydrocarbons in the lean exhaust gas to carbon dioxide and water. They should also oxidize the nitrogen monoxide content of the exhaust gas to nitrogen dioxide, in order that it can react with the basic storage material to give nitrates. In the rich operating phase of the engine which follows, the nitrates formed are decomposed to nitrogen oxides and are reduced with the aid of the catalytically active components, using carbon monoxide, hydrogen and hydrocarbons as reducing agents, to nitrogen with formation of water and carbon dioxide.
In operation, storage catalysts are exposed to very high exhaust gas temperatures at times, which can lead to thermal damage to the catalysts. In the prior art, two main aging effects have been distinguished to date:                The catalytically active noble metal components in the freshly prepared storage catalyst are present in highly disperse form with mean particle sizes between about 2 and 15 nm. Particularly in the lean exhaust gas, an irreversible enlargement of the noble metal crystals is observed with rising exhaust gas temperature. This sintering is accompanied by a significant decrease in the catalytic activity.        The storage components are likewise subject to sintering at high temperatures, which reduces their catalytically active surface area. It has also been observed that the storage components deposited on support materials, at high temperatures, enter into compounds which have a lower storage capacity for nitrogen oxides with the support materials (see SAE Technical Paper 970746 and EP 0982066 A1). When, for example, barium oxide is used as a storage component on a support material comprising cerium oxide, barium cerate (BaCeO3) can be formed.        
The sintering of the noble metal particles or else of the storage components is an irreversible process. Restoration of the original crystal sizes and hence of the original catalytically active surface areas by a specific treatment does not appear to be possible to date. In contrast, components which are lost to the catalytic process because they react with other catalyst constituents to give less active compounds can in principle be recovered when the reaction products of such aging processes are nonvolatile. The prerequisite is that the reaction conditions under which the products formed in the aging reaction can be converted selectively back to the catalytically active starting compounds are known.
For example, barium cerate BaCeO3, which during the thermal aging of nitrogen oxide storage catalysts which comprise barium oxide as a storage component in a cerium oxide-based nitrogen oxide storage material, can be decomposed back to barium oxide and cerium oxide by treatment with a gas mixture comprising nitrogen dioxide, water vapor and optionally carbon dioxide at 300° C. to 500° C. WO 07/009679 to this applicant describes a process for reactivating thermally aged nitrogen oxide storage catalysts, which is based on this operation.
The term “reactivation” in this context should be carefully distinguished from the term “regeneration” which is likewise commonly used in connection with nitrogen oxide storage catalysts.
During the lean operating phase of the engine, nitrogen oxides are stored in the storage material in the form of nitrates. With increasing incorporation, the storage capacity of the material decreases. Therefore, the storage material has to be regenerated from time to time. To this end, the engine is operated with rich air/fuel mixtures for a short time. The interplay of nitrogen oxide storage and regeneration of the storage material results in the cyclic method of operation composed of rich phases and lean phases which is characteristic of this catalyst type, the lean phase typically being 5 to 50 times as long as the rich phase.
The term “reactivation” of the catalyst refers exclusively to the partial restoration of the catalytic activity which has been lost beforehand in a thermal aging process. The thermal aging process is always superposed on the cyclic mode of operation, part of which is the regeneration, when the catalyst is exposed to high operating temperatures. The reactivation is not part of the standard vehicle operation, but instead must, if it can take place in the vehicle at all, be initiated and regulated in a controlled manner as a dedicated operating state by the engine control system of the vehicle. The catalyst can also be reactivated outside vehicle operation, for example during a service. To this end, it may be necessary to deinstall the aged catalyst to be reactivated from the vehicle and to treat it in a device separate from the vehicle.
Such a reactivation process is described in WO 07/009679 to this applicant. As already mentioned, in the process described herein, the catalytic activity of a thermally aged nitrogen oxide storage catalyst which comprises basic strontium or barium compounds on a support material comprising cerium oxide, and additionally comprises strontium and/or barium compounds formed by the thermal aging with the support material—in particular strontium cerate and/or barium cerate—is at least partly restored by treatment with a gas mixture comprising nitrogen dioxide, water vapor and optionally carbon dioxide at 300° C. to 50° C.
The process described in WO 07/009679 does not take account of the aging mechanisms to which the catalytically active noble metal components are subject, since it has been assumed to date that the main aging mechanism for noble metal is the irreversible sintering of the particles. Accordingly, the process described in WO 07/009679 to this applicant can achieve only a partial reactivation of a cerium oxide-based, thermally aged nitrogen oxide storage catalyst.